I had borrowed scopes before! I use it to look into the cylinder bores on race engines to check for piston / bore damage, saves taking the engine a part to check. I also use it after draining the oil in race engines, to check for signs of premature wear of the crank. Also looking inside gear boxes and differentials. Really any mechanical part that is normally removed for visual checking, many can be visually checked with the scope.
Phil Wall, Cambridgeshire, United Kingdom.